Many electronic devices, such as pagers, employ display devices to present information to a user. One such display device that is well known in the art is a liquid crystal display (LCD), which typically displays graphics and alphanumeric characters. The characters are often, however, illegible when the user is in an area that has insufficient lighting. Therefore, a light source is generally mounted behind the LCD so as to illuminate the LCD from within the electronic device.
One conventional method for backlighting an LCD involves the use of an electro-luminescent (EL) panel, which is mounted directly behind the LCD. Typically, an EL panel is constructed from a layer of phosphorous material, on either side of which electrodes are disposed. When a voltage is applied across the electrodes, the EL panel emits a uniformly distributed light by which the LCD is illuminated. The required voltage, however, is typically very high, and, in conventional pagers, step-up circuitry, consisting of at least a transformer, is required to increase the voltage supplied by a battery to the voltage required by the EL panel. The use of an EL panel, therefore, increases the current drain on the battery, thus decreasing the battery life. This decrease in battery life is often unacceptable, as in the case of most pagers which utilize small batteries having low capacities. Furthermore, the commonly large size of the transformer restricts the use of the EL panel in pagers having strict space constraints. In addition to the above mentioned drawbacks, an EL panel is typically very expensive when compared to the cost of other electronic components. The use of an EL panel to illuminate an LCD is, therefore, unsuitable for many conventional pagers.
A further method for backlighting an LCD involves the use of a light wedge, which guides light from one or more light sources, e.g., light emitting diodes (LEDs) or incandescent lamps, to a region behind the LCD. In pagers employing a light wedge, a light source is typically mounted within the pager, and the light wedge is secured between the light source and the LCD. The light wedge is constructed from a translucent material and formed such that light beams emanating from the light source are received by the light wedge and guided, mainly by reflection, towards the LCD. Additionally, a diffuse material typically applied to one of the reflective surfaces of the light wedge further diffuses a portion of the guided light beams. In this manner, the LCD is illuminated from behind by both diffused and direct light supplied by the light source.
Conventionally, the use of a light wedge is much less expensive than the use of an EL panel. Additionally, the combination of a light wedge and light sources typically requires less space than an EL panel and step-up circuitry. Typical light wedges, however, do not commonly provide uniformly distributed light to an LCD. Furthermore, most typical light wedges allow light to pass directly to some regions of the LCD. As a result, "hot spots", brightly lit areas which reveal the location of the light sources, and dim areas, which appear shadowed in comparison to the hot spots, are visible on the surface of the LCD. To overcome this problem, other typical light wedges employ additional light sources to illuminate the dim areas. This solution, however, merely creates a greater number of hot spots if the supplied light is not uniformly distributed. The use of additional light sources also introduces a number of unnecessary and sometimes expensive parts into the design of the electronic device. Therefore, although the light wedge is normally less expensive than an EL panel, the resulting illumination is often poor enough so that the LCD is difficult to read unless additional light sources are utilized.
Thus, what is needed is an improved light wedge which provides a more uniform distribution of light to eliminate hot spots. Furthermore, the light wedge should not require more than a single light source.